Implanted device data to guide ablation therapy

ABSTRACT

A medical device system and associated method for guiding ablation therapy sense cardiac signals using implantable electrodes and detect spontaneous cardiac events from the sensed cardiac signals. Pacing pulses are delivered and a return cycle length is measured in response to the plurality of pacing pulses. The spontaneous cardiac event is clustered with a previously detected cardiac event in response to the measured return cycle length, and a targeted ablation site is estimated in response to the measured return cycle length. A transit time interval, corresponding to a distance traversed by a depolarization associated with a last one of the plurality of pacing pulses when a reset condition occurs, is computed using the return cycle length, and the ablation site is estimated in response to the computed transit time interval.

RELATED APPLICATION

The present disclosure claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 61/426,713, filed Dec. 23, 2010,entitled “IMPLANTED DEVICE DATA TO GUIDE ABLATION THERAPY”, incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to a medical device system and method for producing data forguiding ablation therapy.

BACKGROUND

Cardiac ablation is a therapy used to treat cardiac arrhythmias. Anablation procedure involves identifying abnormal cardiac tissue fromwhich an arrhythmia is arising or creating a circulating pathway forconduction of the arrhythmia then ablating the cardiac tissue toeliminate a focal point of origin or recirculating pathway. The ablationenergy is typically RF ablation though chemical ablation or cyroablationcan be used. Ablation therapy is used to treat patients who experiencearrhythmias that are refractory to medication, experience serious sideeffects from medications used to treat arrhythmias, or experienceserious or life-threatening arrhythmias.

The clinician has the task of properly identifying a targeted site forablation that will successfully eliminate the abnormal tissue. Theprocess of identifying an ablation site can include 12-lead ECG studies,electrical mapping using intracardiac electrodes, and may requireinducing abnormal rhythms in order to identify and confirm an arrhythmiaand an appropriate ablation site. In some cases, the ablation may beincomplete, not entirely eliminating the occurrence of associatedarrhythmia episodes. Some patients may experience more than one type ofarrhythmia, arising from more than one focal point or reentrant circuit.The procedure for identifying the proper ablation site and the correctnumber of ablation sites to satisfactorily reduce the occurrence ofarrhythmias in a given patient is a challenge to the clinician.

Implantable cardiovertor defibrillators (ICDs) are also used to treatpatient's experiencing arrhythmias. ICDs do not eliminate the abnormaltissue causing arrhythmias to occur but can deliver cardiac pacing,anti-tachycardia pacing or cardioversion/defibrillation shocks toprevent or terminate an arrhythmia episode. ICDs typically acquire datarelating to an arrhythmia episode when it does occur, including, forexample, an intracardiac electrogram (EGM) strip, cardiac cycle lengthsmeasured during the episode, EGM signal morphology data, data relatingto the onset of the episode, and therapies delivered to treat anarrhythmia.

Patients receiving ICDs may experience a high occurrence of shocktherapies which are painful and can reduce the quality of life of thepatient. Ventricular ablation can be used in ICD patients thatexperience recurrent ventricular tachycardia (VT) or VT storms to reducethe number of shocks required by the ICD. Data stored by an ICD relatingto sustained or non-sustained VT can include information useful to aclinician performing ablation procedures. Data stored by an ICD can beuplinked to an external ICD programmer for manual review and analysis bya clinician performing an ablation, however the large amount of episodedata can pose considerable data analysis burden on the clinician. Whatis needed therefore, is a system and method that provides a clinicianinformation acquired by an ICD in a useful format to assist theclinician in efficiently and effectively performing an ablationprocedure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an implantable medical device(IMD).

FIG. 2 is a functional block diagram of an ICD system used incombination with an ablation system for guiding ablation therapy.

FIG. 3 is a flow chart of a method for acquiring data for guiding anablation therapy.

FIG. 4 is a flowchart of another method for providing arrhythmia episodedata for guiding ablation therapy.

FIG. 5 is a flowchart of a method for identifying a tachycardia site oforigin during an ablation procedure.

FIG. 6 is a schematic drawing of information included in a cliniciandisplay according to one embodiment.

FIG. 7 is a flowchart of a method for using IMD acquired data and anablation system to localize an ablation site

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments. It is understood that other embodiments may be utilizedwithout departing from the scope of the invention. For purposes ofclarity, the same reference numbers are used in the drawings to identifysimilar elements. As used herein, the term “module” refers to anapplication specific integrated circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and memory that execute one ormore software or firmware programs, a combinational logic circuit, orother suitable components that provide the described functionality.

FIG. 1 is a schematic representation of an implantable medical device(IMD) 10. IMD 10 is embodied as an ICD in FIG. 1. Methods describedherein, however, should not be interpreted as being limited to anyparticular IMD. Instead, embodiments may include any IMD so long as thedevice utilizes electrodes for monitoring the cardiac rhythm of apatient by sensing cardiac EGM signals, with or without therapy deliverycapabilities.

In FIG. 1, the right atrium (RA), left atrium (LA), right ventricle(RV), left ventricle (LV), and the coronary sinus (CS), extending fromthe opening in the right atrium to form the great cardiac vein, areshown schematically in heart 8. Two transvenous leads 16 and 18 connectIMD 10 with the RV and the LV, respectively. A third transvenous lead 4is positioned in the RA. Each lead includes at least one electricalconductor and pace/sense electrode. For example, leads 16 and 18 arerespectively connected to pace/sense electrodes 20, 22, and 14, 15. RAlead 4 carries pace/sense electrodes 5 and 7. In addition, a housingelectrode 12 can be formed as part of the outer surface of the housingof the device 10. The pace/sense electrodes 5, 7, 14, 15, 20, 22 andhousing electrode 12 can be selectively employed to provide a number ofunipolar and bipolar pace/sense electrode combinations for pacing andsensing functions. The depicted positions in or about the right and leftheart chambers are merely illustrative. Moreover, other leads andpace/sense electrodes can be used instead of, or in combination with,any one or more of the depicted leads and electrodes.

The electrodes designated herein as “pace/sense” electrodes may be usedfor both pacing and sensing functions. In other embodiments, theseelectrodes can be used exclusively as pace or sense electrodes inprogrammed or default combinations for sensing cardiac signals anddelivering pacing pulses. The leads and electrodes described can beemployed to record cardiac signals. The recorded data can beperiodically transmitted to a programmer or other external deviceenabled for telemetric communication with the IMD 10. As will bedescribed in detail below, pace/sense electrodes will be used incollecting data relating to arrhythmia episodes for use in guidingablation therapy.

An RV coil electrode 21 and a superior vena cava (SVC) coil electrode 23are also shown as being coupled to a portion of RV lead 16. Coilelectrodes can additionally or alternatively be coupled to portions ofRA lead 24 or CS lead 18. The coil electrodes 21 and 23, or othersimilar electrode types, can be electrically coupled to high voltagecircuitry for delivering high voltage cardioversion/defibrillation shockpulses and optionally for collecting cardiac electrical data relating toarrhythmia episodes for use in guiding ablation therapy.

Electrodes shown in FIG. 1 can be disposed in a variety of locations in,around, and on the heart and are not limited to the locations shown.Furthermore, other lead and electrode systems may be substituted for thesystem shown in FIG. 1, including electrodes placed on or around the IMDhousing. The system and associated methods described herein may includethe use of electrodes for sensing atrial signals and ventricular signalsfor detecting cardiac arrhythmias and for clustering arrhythmia episodesaccording to common characteristics. In other embodiments, singlechamber, dual chamber or multi-chamber systems may be used which includeone or more leads used to position electrodes in, on or around the heartchambers.

FIG. 2 is a functional block diagram of an ICD system used incombination with an ablation system for guiding ablation therapy.Circuitry 24, located within IMD 10 of FIG. 1, includes pacing circuitry25, defibrillation circuitry 26, sensing circuitry 27, control processor28, memory 29, and communication system 30. Leads 4, 16 and 18 areconnected to pacing circuitry 25, defibrillation circuitry 26 andsensing circuitry 27. Each lead (and in turn individual electrodesassociated with each lead) coupled to the IMD may be used in multiplecapacities to sense cardiac depolarizations (e.g. P-waves and R-waves),deliver pacing pulses including anti-tachycardia pacing (ATP) pulses,and deliver defibrillation or cardioversion shocks.

Control processor 28 receives input through sensing circuitry 27 fromleads 4, 16 and 18 concerning depolarizations sensed by the electrodesconnected to leads 4, 16 and 18. Based on input received from sensingcircuitry 27, control processor 28 performs an arrhythmia detectionalgorithm for detecting arrhythmias and selecting a therapy as needed.Therapy may include providing ATP therapy pacing electrodes using pacingcircuitry 25 and selected pacing electrodes, providing defibrillation orcardioversion shocks using defibrillation circuitry 26 and a selectedhigh voltage electrode, or providing no treatment at all.

Control processor 28 stores selected data to memory 29, and retrievesstored data from memory 29 as necessary. As will be described in detailherein, control processor 28 extracts characteristics from arrhythmiaevents and stores these characteristics in memory 29. Processor 28 mayfurther cluster or group arrhythmia events based on a correlationbetween extracted characteristics and store the clustered arrhythmiaevents in memory 29.

Communication system 30 includes telemetry processor 31, transmissioncircuitry 32, receiving circuitry 33, and antenna 34. Communicationsystem 30 allows communication between IMD 10 and devices external tothe patient. IMD 10 is shown in bi-directional telemetric communicationwith an external patient management system 40, which in turn is incommunication with an ablation system 42. Additionally or alternatively,IMD 10 may be in direct communication with the ablation system 42.

In one embodiment, external patient management system 40 includes aprogrammer that is used at bedside or in a clinical setting forinterrogating IMD 10 to retrieve data stored in memory 29 and forprogramming operating parameters in IMD 10. External patient managementsystem 40 may include or be embodied as a home monitor that retrievesdata from IMD 10 in a patient's home for use in remote patientmonitoring. External patient management system 40 may include acommunication network linking a programmer or home monitor to a databaseaccessible by a clinician for reviewing retrieved patient data andcurrent IMD operating status. A networked patient management system 40allows data and information collected by IMD 10 relating to arrhythmiasexperienced by the patient to be transferred to an ablation system 42.

External patient management system 40 may transmit data to the ablationsystem 42 for use in guiding an ablation therapy. External patientmanagement system 40 may receive data or instructions from the ablationsystem to allow cooperative acquisition and interpretation of databetween the two systems 40 and 42 for guiding an ablation therapy.Received instructions may include instructions for performing particulardiagnostic testing using IMD 10 prior to, during or after an ablationprocedure. Alternatively, the ablation system 42 is in directcommunication with IMD 10 for retrieving data and/or transferringinstructions for guiding an ablation procedure.

Ablation system 42 includes a user display 46, an ablation source 47,and a mapping source 48 which are adapted to be coupled to an ablationcatheter and mapping lead(s) 44. The mapping source 48 delivers pacingpulses to mapping electrodes positioned on one or more mapping leads 44and pace map data is displayed on display 46. Mapping source may alsodeliver stimulation pulses for inducing tachycardia or fibrillation toallow activation maps to be acquired during an arrhythmia. In otherembodiments, the ablation system 42 may include 12-lead ECG sensing foruse in identifying the anatomic location of the origin or pathway of atachycardia.

The ablation source 47 generates ablative energy delivered by theablation catheter 44 to a targeted ablation site identified using dataprovided by the IMD and stored in memory 29 relating to arrhythmiaepisodes experienced by the patient. The IMD 10, external patientmanagement system 40, and ablation system 42 may therefore be configuredto function cooperatively to acquire data and information that isdisplayed in a common image on display 46 to guide the ablationprocedure.

Display 46 may be a graphical user interface that allows the clinicianto select different screens of information and select actions orfunctions to be performed by the system. For example, one screen orwindow may list summary data acquired by the IMD relating to arrhythmiaepisodes potentially targeted for ablation therapy. Another screen orwindow may provide an image of the patient's heart, activation maplocations and associated EGM/ECG morphology. Registered locations ofintracardiac pace/sense electrodes (coupled to the IMD), mappingelectrodes, and/or the ablation electrode may be superimposed on amapping image.

FIG. 3 is a flow chart 100 of one method for acquiring data for guidingan ablation therapy. At block 102, an arrhythmia event or episode isdetected. Arrhythmia episodes may include all ventricular arrhythmias,atrial arrhythmias, and double tachycardia episodes (occurring in bothatrial and ventricular chambers). Events or episodes detected at block102 may include premature ventricular contractions (PVCs), sustainedarrhythmias and non-sustained arrhythmias and may be events that aretreated or untreated by the IMD. Events may be triggering events orprecursors that precede a tachycardia or fibrillation episode.Triggering events may be a PVC, a non-sustained arrhythmia episode, orother accelerated rhythm.

At block 104, episode parameters are extracted. Episode parameters mayinclude any parameters used to detect the event or episode and mayfurther include parameters extracted after detection for use incharacterizing the detected event or episode. Episode parameters mayinclude chamber of origin, mean cycle length, cycle length regularity,representative morphology of the cardiac signal, morphology matchingscore to a known morphology template, regularity of morphology matchingscores relative to a known template or relative to matching successivebeats to each other during a tachycardia, measurements relating to theepisode onset and/or offset, timing correlation to a referenceelectrogram (such as a subcutaneous or surface ECG), and A-V sensedevent patterns if available. When the detected event is an individualbeat, like a PVC, a representative morphology and coupling interval maybe stored.

Other sensor signals besides cardiac EGM or subcutaneous ECG signals mayalso be used for extracting episode parameters. For example if a bloodpressure sensor, oxygen sensor, heart wall motion sensor or other sensorproviding a signal correlated to patient hemodynamic status isavailable, a parameter corresponding to hemodynamic stability of thepatient during a detected episode may be extracted as an episodeparameter.

The episodes are clustered into groups of episodes characterized bysimilar parameter values at block 106. The clustering operation may bethought of as plotting detecting episodes in a multi-dimensional plotwith each dimension representing an episode parameter. Episodes havingsimilar characteristics will appear as clusters in the multi-dimensionalplot. In an illustrative example, 50 episodes may be detected in apatient over a period of time. Of those episodes, five episodes may beSVT episodes and the remaining 45 may be VT episodes. Of the 45 VTepisodes, three distinct episode types may be identified as a result ofthe clustering process, e.g. due to differences in morphology, cyclelength, onset or other extracted parameters.

A cluster of VT episodes may be further indicated as most likely being afocal VT episode or a reentrant VT based on cycle length, therapysuccess, EGM morphology regularity or other parameters. For example, afocal VT may be less regular in cycle length and EGM morphology frombeat-to-beat and may be more likely to spontaneously terminate. Knowingwhether the VT is focal in origin or a reentrant VT will be useful to aclinician in guiding the ablation procedure.

If a particular episode type reaches an alert threshold at block 108, aalert is generated at block 110 to notify a clinician or patient thattherapeutic intervention may be warranted. The alert threshold may beprogrammable and may be defined as a total number of episodes in asingle cluster or a group of clusters. The alert threshold may includetime dependency such that the frequency of episodes occurring in aparticular cluster must increase to reach an alert threshold level. Analert threshold generated by an ICD may indicate that the episode typeoccurs with such a high degree of frequency that the patient may be acandidate for ablation therapy. The alert threshold may be programmed bythe implanting physician, then viewable by a primary physician so thatthe primary physician understands that an electrophysiological (EP)limit has been reached and a referral back to an EP specialist iswarranted.

At block 112, cluster summary data is displayed to a clinician. Theprocess shown in flowchart 100 may be implemented in a distributedmanner across an IMD/ablation system. The IMD may detect arrhythmiaepisodes at block 102 and the episode may be stored by the IMD. Episodeparameters and episode clustering may then be performed by an externalprocessor in external patient management system 40 or ablation system 42using episode data retrieved from the IMD. The cluster summary data maythen be displayed to a clinician upon request, by an external patientmanagement system 40 associated with the IMD or by an ablation system42. Alternatively, the episode detection, parameter extraction, andepisode clustering may be performed by the IMD. The episode cluster datamay then be transmitted to an external system component 40 or 42 fordisplay to a clinician.

In some embodiments, the episode cluster data, after being uplinked toan external device such as a remote patient management network, may bedownloadable in a transferrable electronic data format for emailtransmission or display on a secure website to be reviewed by a clinicalspecialist prior to scheduling a clinical consultation.

The displayed information can then be used by the clinician to guide anablation therapy. For example, the clinician will be aware if thepatient is experiencing more than one type of VT episode. The clinicianis able to target the most clinically important arrhythmias based on thecluster data provided, e.g. the episode clusters having the shortestcycle length, occurring with greatest frequency, or most hemodynamicallyunstable.

Flow chart 100 is intended to illustrate the functional operation of thedevice, and should not be construed as reflective of a specific form ofsoftware or hardware necessary to practice the methods described. It isbelieved that the particular form of software, hardware and/or firmwarewill be determined primarily by the particular system architectureemployed in the device and by the particular detection and therapydelivery methodologies employed by the device. Providing software,hardware, and/or firmware to accomplish the described functionality inthe context of any modern IMD and ablation system, given the disclosureherein, is within the abilities of one of skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

FIG. 4 is a flowchart 200 of another method for providing arrhythmiaepisode data for guiding ablation therapy. In this method, a returncycle length (RCL) is measured for use in clustering detected episodesand for guiding a clinician to an ablation site. For details regardingthe concepts associated with measuring RCL, reference is made tocommonly-assigned U.S. Pat. Publication No. 2010/0198290 (Jackson etal.), hereby incorporated herein by reference in its entirety.

At block 202, a spontaneous tachycardia episode is detected. Tachycardiadetection may be performed according to any detection algorithmimplemented in the IMD, including rate- and morphology-based algorithms.The episode may be detected using any available sensing electrodes. Atblock 204, the cycle length of the tachycardia is measured and is storedas an episode characteristic for use in clustering episodes. Thetachycardia cycle length (TCL) is further used to determine a pacingcycle length for an ATP regimen such that the ATP pulses are deliveredat cycle lengths that are shorter than the TCL.

At block 206, an initial ATP therapy is delivered. The number of pulsesand the total prematurity of the initial ATP regimen may be deliveredaccording to any programmed settings, e.g. according to a first level oftherapy in a tiered menu of therapies. The “prematurity” of a single ATPpulse is the difference between that pulse's ATP cycle length and theTCL. The total prematurity of the ATP regimen is the sum of all of theATP pulse prematurities in an ATP sequence. The initial ATP therapy maybe a burst, ramp, ramp plus burst or any other ATP regimen.

Following the last pulse of the ATP therapy, the RCL is measured atblock 208. The methods generally disclosed in the above-incorporated'290 reference may be used for measuring RCL and computing a transittime as generally discussed below. The RCL is the time between deliveryof the last pacing pulse of an ATP sequence and the first sensed eventoccurring after the last pacing pulse.

At block 210, the RCL is analyzed to determine if reset is detected.When timed appropriately during the TCL, an ATP depolarization wavefrontwill be injected ahead of a circulating reentrant wavefront of thetachycardia. This phenomenon is referred to as “reset” in that the ATPdepolarization has altered a cycle length within the tachycardiacircuit. If the injected ATP depolarization is the final pulse of an ATPregimen, the next depolarization sensed at the electrode site will bethe injected ATP depolarization after it travels around reentrantcircuit and back to the pace/sense electrode. The RCL will be shorterthan the expected TCL because the injected ATP depolarization has“jumped in line” ahead of the next expected tachycardia depolarization.

Accordingly, a measured RCL that is less than the sum of the measuredTCL and the total prematurity of the ATP sequence is an indication thatreset has occurred. Reset is detected at block 210 based on a comparisonof the measured RCL to the TCL and total prematurity. If the RCL isapproximately equal to (within predetermined uncertainty limits) orgreater than the sum of the total prematurity plus the TCL, reset is notdetected. Uncertainty limits may take into account a known or unknownvariation in the TCL. If reset is not detected, another ATP regimen isdelivered at block 214 with a greater total prematurity than the initialATP regimen.

When reset occurs, i.e. when the RCL is less than the TCL plus the totalprematurity of the ATP pulses, the last pulse of the ATP regimen hastraveled the distance between the pace electrode and the tachycardiasite of origin. The RCL observed after a reset allows a directmeasurement of the transit time (TT) which is correlated to thedifference between the RCL at which reset occurs and the TCL. The TT isdirectly correlated to the distance from a pace/sense electrode to thesite of origin of the tachycardia. Transit time can be used todiscriminate between SVT and VT as well as help to localize a site oftachycardia origin for guiding an ablation therapy.

Once reset is detected, the TT corresponding to the distance between thestimulating electrode and the tachycardia origin is computed at block212. The TT is stored as an episode characteristic at block 216. Whendetected episodes are clustered at block 218, after each detectedepisode, periodically or upon request by a user, the measured TCL, RCL,and TT, or any combination thereof, may be used as episodecharacteristics during the clustering process.

While not explicitly shown in FIG. 4, it is recognized that a limitednumber of ATP sequences may be delivered in an attempt to detect reset.If reset is not detected, even after adjusting the total prematurity ofthe ATP sequence, a minimum distance from the stimulating electrode tothe tachycardia site of origin can still be estimated based on the RCLand the maximum total prematurity of the attempted ATP sequences. Thedistance corresponds to a time that is longer than the sum of themeasured RCL and total prematurity. An ATP therapy delivered in order tomeasure a RCL may or may not terminate the tachycardia.

After assigning an episode to a cluster using the measured TT at block218, the number of episodes in each cluster can be compared to an alertthreshold at block 220. As described above, if an alert threshold ismet, an alert is generated at block 220 to notify a clinician or patientthat therapeutic intervention may be warranted. At block 224, clustersummary data obtained using RCL measurements is displayed to a clinicianupon user request.

FIG. 5 is a flowchart 300 of a method for identifying a tachycardia siteof origin during an ablation procedure. At block 302, tachycardia isinduced. The cycle length of the induced tachycardia is measured atblock 304. ATP is delivered at block 306 to allow a RCL to be measuredat block 308. The ATP is delivered using the same pace electrode as usedfor delivering ATP when the patient experiences spontaneous arrhythmias.

The ATP regimen delivered at block 306 may be selected based on a matchbetween the TCL and an episode cluster identified from the IMD data. Ifthere is a match between a TCL of an episode cluster and the inducedTCL, the ATP sequence found to reset the episode type may be used atblock 306. Alternatively, a default ATP sequence may be delivered.Adjustments to the ATP sequence may be made at block 314 until reset isdetected at block 310.

The TT is computed at block 312 using the RCL and the measured TCL. TheTT is compared to episode clusters at block 316 to determine if a matchis identified. Other episode parameters may also be used such as TCL,morphology parameters or any of those listed previously.

An episode cluster matching the induced TCL and measured TT isidentified from episode cluster data retrieved from the IMD at block318. A display may be generated at block 318 indicating an approximatelocation of the tachycardia site of origin relative to the stimulatingelectrode. If the induced VT matches a cluster, the VT is concluded tocorrespond to a clinical VT experienced by the patient, which may thenbe targeted for ablation therapy. The site of tachycardia induction andthe site of intrinsic tachycardia origin need not be the same to producethe same type of VT. If the VT is hemodynamically stable, mapping can beperformed during the VT to identify a location within the VT circuit totarget for ablation.

At block 320, a potential ablation site is identified using anytechnique preferred by the clinician such as pace mapping, ECGmorphology analysis, VT entrainment or other method. The ablationcatheter is advanced to position an ablation electrode at the potentialsite.

A pacing pulse is delivered at block 322 using the pace electrodecoupled to the IMD and used by the IMD for measuring RCL and TT ingenerating episode cluster data. Using the ablation electrode at thepotential ablation site, the evoked potential is sensed at the potentialablation site in response to the pacing pulse. The transit time for thepacing pulse to travel to the potential ablation site is measured atblock 324. If this transit time matches the TT associated with anepisode cluster targeted for therapy, as determined at decision block326, the potential site is confirmed as a targeted ablation site atblock 328. Ablation energy is delivered at block 330 to treat theepisode type. If the TT does not match an episode cluster TT asdetermined at block 326, the process returns to block 320. The ablationelectrode may be repositioned to a new potential ablation site. Futureepisode cluster data may be acquired for confirming successful treatmentof the episode type. In other words, absence of future episodes matchingthe data cluster representing the treated episode type would indicatetreatment success.

FIG. 6 is a schematic drawing of information included in a display 350according to one embodiment. Display 350 may be implemented in anablation system 42 (shown in FIG. 1) incorporating data received fromthe IMD 10 directly or via external patient management system 40. Oncethe distance from the pacing electrode to the tachycardia origin isapproximately known based on TT, an activation map can be generatedduring cardiac pacing. On a display 350, the location of the IMD paceelectrode 352, typically an RV pace electrode is superimposed on animage of the patient's heart 351 or portion thereof. An ablation siteguidance line 354 is displayed marking an approximate distance from thepacing electrode 352 corresponding to the measured TT 355 of an episodecluster. If a transit time is not measured because reset was notdetected, a guidance line 354 may mark a boundary indicating a minimumTT (or distance) to the tachycardia circuit based on a RCL at whichreset did not occur.

A pacing pulse can be delivered by the IMD pace electrode 352 and anactivation map may be generated from a single pacing pulse using, forexample, non-contacting imaging systems, such as EnSite 3000,Endocardial Solutions Incorporated, St. Paul, Minn. The activation mapis superimposed with the ablation site guidance line 354, with mappinglocations 356 through 358 and 361 through 363 indicated by symbols.Corresponding EGM morphologies 360 for the map locations 356-358 and361-363 may be shown in the display 350.

The clinician can focus attention on the activation map locations 358and 362 that are in close proximity to the ablation guidance line 354that is associated with a resetting of a tachycardia circuit. Asdescribed above, a potential ablation site may be confirmed by sensingthe evoked response at the potential site using an ablation electrode(or mapping electrode) placed at the site and measuring the actualtransit time of an IMD pace electrode pulse to the potential site. Ifthe measured TT approximately matches the stored TT for a VT episodecluster being targeted for treatment, the ablation site is confirmed.

A VT episode that is hemodynamically unstable may be considered“unmappable”. Using the activation guidance line 354, a VT previouslyconsidered unmappable may be mapped by focusing only on a narrow regioncorresponding to the guidance line 354 to quickly obtain a regionalizedactivation map allowing early termination of the induced VT.

In some embodiments, one or more additional ventricular pace electrodesmay be used to deliver pacing pulses for measuring TT during a repeatedepisode of the same type, or sequentially or simultaneously during thesame episode, to estimate the distance to the VT circuit from adifferent pace pulse delivery location. In this case, a second electrodelocation may be displayed with a corresponding second ablation guidanceline. An intersection or common region defined by the two (or more)guidance lines may aid in better localizing of the VT circuit.

FIG. 7 is a flowchart 400 of a method for using IMD acquired data and anablation system in delivering an ablation therapy. It is understood thatthe various blocks shown in FIG. 7 and in other flowcharts describedherein may be arranged in other orders than the order shown and someblocks may be optional or omitted. At block 401, a summary of episodecluster data accumulated by the IMD is displayed, either on an externalpatient management system 40 or ablation system 42. This summary mayinclude a listing of the number of episode types according to episodecluster data and may be ordered according to frequency of occurrence,potential severity (e.g. TCL or hemodynamic instability), number ofdelivered therapies by the IMD and associated success rate, or otherhierarchical ranking. The clinician may select an episode cluster in thelist to review more detailed data about a particular episode type.

At block 402, an image representing the patient's heart is displayed.The location of intracardiac pace electrode(s) used to measure a TT (orRCL) for one or more associated episode clusters is also shown in thedisplay. When TT data is available for an episode cluster, an ablationguidance line is also displayed relative to the associated paceelectrode, as generally described in conjunction with FIG. 6. In somecases, ablation at a single site may treat more than one type of VTepisode. Simultaneous display of multiple guidance lines may enable aclinician to identify a potential ablation site that may reduce morethan one type of VT episode simultaneously.

At block 404, tachycardia is induced to confirm diagnosis and indicationof ablation therapy for an inducible VT episode. At block 406 mapping isperformed. If an induced VT is hemodynamically stable and tolerated bythe patient, mapping may be performed during the VT. If not, substratemapping of the ventricle(s) is performed at block 406. Mappingprocedures and preferences will vary between clinical centers. Theclinician will have the episode cluster data from the IMD available andbe aware of the number of different episode types potentially targetedfor treatment by ablation during the mapping procedure. The mappingprocedure may variously involve delivering electrical pulses to thepatient's heart for inducing tachycardia, performing pace mapping,entrainment mapping, and/or studying surface ECG signals at blocks 404and 406. Using relative timing differences from EGM signals, RCL, TT orother information, the clinician will have an initial indication of anapproximate location of a triggering event site or a tachycardia circuitsite. The clinician may use this knowledge in keying in on theactivation time and electrogram signals of particular anatomical areas.

The mapping procedure may be performed in a cooperative manner betweenthe IMD system and the ablation system. The IMD system may remainprogrammed to detect an induced arrhythmia and extract parameters foridentifying a matching cluster of episode data. The IMD system mayadditionally be enabled to initiate an ATP sequence for measuring RCLand TT if reset is detected.

During mapping at block 406, intracardiac EGM signals obtained by theIMD may also be recorded and analyzed. One or more EGM sensing vectorsmay be sensed to allow differential timing information to be gathered.At block 408, characteristics used to cluster episode data are extractedfrom the EGM signal(s). For example, a representative morphology, cyclelength, slew rate, or other features may be determined. Thesecharacteristics are compared to corresponding characteristics of storedepisode clusters at block 410 to determine if the clinician has induceda tachycardia that correlates to an episode cluster. If a matchingcluster is found, the clinician has identified a VT that corresponds toa VT episode cluster identified by the IMD. If no match is found, theinduced VT is evaluated clinically by the physician at block 414according to individual practice.

If a match if found, a decision is made at block 422 if the currentlymeasured EGM characteristics correlate to trigger events stored in theepisode cluster data. For example, the morphology of the mapped signalsmay correspond to the morphology of a VT cluster associated with a PVCor non-sustained VT identified as a triggering event for a sustained VTcluster. In this case, the location of the triggering event origin maybe approximated using the 12-lead ECG signals or other mapping methods.Relative timing information between two or more EGM vectors may be usedto further localize the triggering event origin. In a display, thelocation of the EGM leads and the estimated location of the triggeringevent origin may be shown at block 428. With this information, theclinician can identify and ablate the triggering event origin at block430.

If there are no more episode clusters identified by the IMD fortreatment, as determined at block 448, the process is stopped at block450. Ongoing episode cluster data will provide information as to whetherthe targeted cluster of episodes has been successfully treated or not.

If an associated triggering event is not identified, the matchingcluster identified at block 412 may be used to identify the tachycardiasite of origin using both mapping data and IMD cluster data at blocks432 and 434. A clinician will often have a general idea of a location ofa tachycardia site of origin, either focal or re-entrant forms oftachycardia, through observation of the 12-lead ECG signals or usingother mapping methods. The cluster data provides the clinician withadditional information for more precisely localizing a site of origin,for example using the TT data at block 434, as described above.

After matching a mapped signal morphology with a stored episode cluster,the TT data stored for the matching cluster is retrieved at block 434and used to display an approximate distance of the resetting circuitfrom a pacing electrode on the ablation image at block 436. In this way,matching morphology or timing characteristics are used to identify amatching episode cluster and then stored RCL and TT data for thematching episode cluster are used directly for indicating a site oforigin without performing additional RCL or TT measurements during themapping procedure. It is recognized that in other embodiments, the RCLand TT measurements may be repeated during the mapping study to verify amatch between a stored episode cluster and the induced VT or to confirman ablation electrode location based on a measured TT as described inconjunction with FIG. 5.

The EGM lead/electrode locations and estimated site of origin based oncluster data, e.g. an ablation guidance line based on a TT measurementas described above, can be displayed in the mapped image at block 436 toguide the clinician in ablating the tachycardia origin or pathway atblock 438. If there are more stored episode clusters identified by theIMD for potential ablation therapy as determined at block 448, theprocess returns to block 404 to continue the induction and mappingprocedures. If not the process is terminated at block 450. Ongoingepisode cluster data acquisition by the IMD will provide evidence of thedegree of success of the ablation procedure.

Thus, a system and associated method for guiding ablation therapy havebeen presented in the foregoing description with reference to specificembodiments. It is appreciated that various modifications to thereferenced embodiments may be made without departing from the scope ofthe invention as set forth in the following claims.

1. A medical device system for guiding a cardiac ablation therapy,comprising; a plurality of implantable electrodes for sensing cardiacsignals; an implantable cardiac event detector coupled to the pluralityof implantable electrodes for detecting spontaneous cardiac events fromthe sensed cardiac signals; an implantable pulse generator coupled tothe plurality of implantable electrodes for delivering pacing pulses; animplantable controller to control the pulse generator to deliver aplurality of pacing pulses in response to detecting a spontaneouscardiac event using a first electrode of the plurality of implantableelectrodes and measure a return cycle length in response to theplurality of pacing pulses; and a processor configured to cluster thespontaneous cardiac event with a previously detected cardiac event inresponse to the measured return cycle length, wherein the controller isfurther configured to compute a transit time interval using the returncycle length, the computed transit time interval corresponding to adistance traversed by a depolarization associated with the last one ofthe plurality of pacing pulses when a reset condition occurs, andwherein the processor is further configured to estimate a targetablation site in response to the computed transit time interval.
 2. Thesystem of claim 1, further comprising an external display to display theestimated ablation site for the clustered cardiac events.
 3. The systemof claim 1, wherein the controller is further configured to measure acardiac event cycle length in response to the detected tachycardiaevent, compute a total prematurity of the plurality of pacing pulses,detect a reset condition when the return cycle length is less than a sumof the cycle length and the total prematurity, and determine the transittime interval in response to detecting the reset condition.
 4. Thesystem of claim 1, further comprising an ablation system, the ablationsystem comprising: a cardiac electrophysiology mapping system; anablation electrode; and an ablation source coupled to the ablationelectrode, wherein the display displays the estimated ablation sitesuperimposed with electrophysiology mapping results produced by themapping system.
 5. The system of claim 1, wherein the ablation system,the controller, and the processor are further configured tocooperatively induce a tachycardia, compare the induced tachycardia to acluster of spontaneous cardiac events and identify a matching cluster ofspontaneous cardiac events in response to the comparison, the estimatedablation site corresponding to the matching cluster of spontaneouscardiac events.
 6. The system of claim 1, further comprising an ablationsystem, the ablation system comprising: a cardiac electrophysiologymapping system; an ablation electrode; and an ablation source coupled tothe ablation electrode, the ablation system, the controller and theprocessor configured to cooperatively induce a tachycardia, deliver aplurality of pacing pulses in response to inducing the tachycardia,measure a transit time interval at the estimated ablation site using theablation electrode positioned at the estimated ablation site and confirmthe estimated ablation site in response to a comparison of the computedtransit time interval and the measured transit time interval.
 7. Thesystem of claim 6, wherein the processor is further configured tocompare the induced tachycardia to a cluster of spontaneous cardiacevents and identify a matching cluster of spontaneous cardiac events inresponse to the comparison, and wherein the controller is configured todeliver a plurality of pacing pulses in response to detecting theinduced tachycardia having a total prematurity known to result in areset of the matching cluster of spontaneous cardiac events.
 8. Thesystem of claim 4, wherein the ablation source is controlled to deliverablation energy at the estimated ablation site.
 9. The system of claim4, wherein the estimated ablation site corresponds to a triggering eventlocation of the clustered spontaneous cardiac events.
 10. The system ofclaim 1, wherein the processor is configured to generate an alert when acluster of spontaneous cardiac events reaches an alert thresholdindicating ablation therapy.
 11. The system of claim 1, furthercomprising a communication system coupled to a network for transferringthe data corresponding to the clustered cardiac events in an electronicformat to a data destination.
 12. A method for guiding a cardiacablation therapy in a medical device system, comprising; sensing cardiacsignals using a plurality of implantable electrodes; detectingspontaneous cardiac events from the sensed cardiac signals using animplantable cardiac event detector coupled to the plurality ofimplantable electrodes; delivering pacing pulses using a first electrodeof the plurality of implantable electrodes using an implantable pulsegenerator coupled to the implantable electrodes in response to detectinga spontaneous cardiac event; measuring a return cycle length in responseto the plurality of pacing pulses; clustering the spontaneous cardiacevent with a previously detected cardiac event in response to themeasured return cycle length; estimating a targeted ablation site inresponse to the measured return cycle length; computing a transit timeinterval using the return cycle length, the computed transit timeinterval corresponding to a distance traversed by a depolarizationassociated with a last one of the plurality of pacing pulses when areset condition occurs; and estimating the ablation site in response tothe computed transit time interval.
 13. The method of claim 12, furthercomprising displaying the estimated ablation site for the clusteredcardiac events.
 14. The method of claim 12, further comprising:measuring a cardiac event cycle length in response to the detectedtachycardia event; computing a total prematurity of the plurality ofpacing pulses; detecting a reset condition when the return cycle lengthis less than a sum of the cycle length and the total prematurity; anddetermining the transit time interval in response to detecting the resetcondition.
 15. The method of claim 12, further comprising performing acardiac electrophysiology mapping procedure using an ablation system,the ablation system comprising: a mapping system: an ablation electrode;and an ablation source coupled to the ablation electrode, the methodfurther comprising displaying the estimated ablation site superimposedwith electrophysiology mapping results produced by the mapping system.16. The method of claim 15, further comprising: inducing a tachycardia;comparing the induced tachycardia to a cluster of spontaneous cardiacevents; identifying a matching cluster of spontaneous cardiac events inresponse to the comparison; and displaying the estimated ablation sitecorresponding to the matching cluster of spontaneous cardiac events. 17.The method of claim 12, further comprising: inducing a tachycardia;delivering a plurality of pacing pulses using the first electrode inresponse to inducing the tachycardia; measuring a transit time intervalat the estimated ablation site using the ablation electrode positionedat the estimated ablation site; and confirming the estimated ablationsite in response to a comparison of the computed transit time intervaland the measured transit time interval.
 18. The method of claim 17,further comprising: comparing the induced tachycardia to a cluster ofspontaneous cardiac events; identifying a matching cluster ofspontaneous cardiac events in response to the comparison; and deliveringthe plurality of pacing pulses in response to detecting the inducedtachycardia having a total prematurity known to result in a reset of thematching cluster of spontaneous cardiac events.
 19. The method of claim15, further comprising controlling the ablation source to deliverablation energy at the estimated ablation site.
 20. The method of claim15, wherein the estimated ablation site corresponds to a triggeringevent location of the clustered spontaneous cardiac events.
 21. Themethod of claim 12, further comprising generating an alert when acluster of spontaneous cardiac events reaches an alert thresholdindicating ablation therapy.
 22. The system of claim 12, furthercomprising a communication system coupled to a network for transferringthe data corresponding to the clustered cardiac events in an electronicformat to a data destination.
 23. A computer-readable medium storing aset of instructions which cause a medical device system to perform amethod, the method comprising: sensing cardiac signals using a pluralityof implantable electrodes; detecting spontaneous cardiac events from thesensed cardiac signals using an implantable cardiac event detectorcoupled to the plurality of implantable electrodes; delivering pacingpulses using a first electrode of the plurality of implantableelectrodes using an implantable pulse generator coupled to theimplantable electrodes in response to detecting a spontaneous cardiacevent; measuring a return cycle length in response to the plurality ofpacing pulses; clustering the spontaneous cardiac event with apreviously detected cardiac event in response to the measured returncycle length; estimating a targeted ablation site in response to themeasured return cycle length; computing a transit time interval usingthe return cycle length, the computed transit time intervalcorresponding to a distance traversed by a depolarization associatedwith a last one of the plurality of pacing pulses when a reset conditionoccurs; and estimating the ablation site in response to the computedtransit time interval.